This invention relates generally to preloaded bearings as used in spindles, axles or shafts and, more particularly, to a preload adjustment apparatus for use with preloaded bearings.
In order for a machine tool spindle to maintain high accuracy under varying load conditions, the spindle must have a high degree of stiffness. Stiffness is generally obtained by using angular contact ball bearings mounted in pairs such that the bearings are preloaded against each other. That is, a thrust load is applied such that the bearing balls are forced tightly against the raceways.
Preloading the bearings, to obtain the required stiffness, increases the stresses in the bearings, causing heat generation. This reduces bearing life and limits operating speed. By varying the bearing preload according to need, bearing life and operating speed can be improved. A mechanical apparatus for varying the preload is described in U.S. Pat. No. 4,657,412.
Preload can also be varied using a piezoceramic actuator as described in U.S. Pat. No. 4,850,719. As disclosed therein, a D.C. potential is applied to interfaces of multiple piezoelectric wafers in response to varying load conditions to control bearing stiffness. However, because current piezoelectric actuator technology produces only small displacements, the ability to compensate for thermal expansion and control stiffness is severely limited.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.